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    <script>
      /* 
        =
        可以将符号右侧的值赋值给符号左侧的变量
        +=
        a += 5等价于a = a+5
        a -= 5等价于a = a-5
        a *= 5等价于a = a*5
        a /= 5等价于a = a/5
        */
      var a = 10;
      a += 5;
      console.log("a=" + a);
    </script>
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